1. Wheelchair Seating System Design Considerations
Patients who are immobilized for any number of reasons are prone to develop pressure sores or decubitus ulcers. It is highly desirable to design wheelchair seating systems for such patients which minimizes this problem. The problem of pressure sores is especially severe at any point on the body where a load-bearing bone is positioned adjacent to an underlying support medium. Pressure sores are areas of necrotic tissue resulting from localized pressure concentration in an area of the body from prolonged contact with a chair or a bed. Patients who are victims of stroke, hip fracture, spinal cord injury, and head injury, as well as paraplegics and geriatrics using wheelchairs, are particularly susceptible to pressure sores.
The area in which pressure sores develop on the patient's body is very localized. It has been estimated, that when seated on a flat surface, up to 75% of a person's body weight is distributed over an area covering only four square inches, usually in the region of the ischia or seat bones. For a person weighing 150 pounds, this means as much as 110 pounds of pressure is distributed over just four square inches, or over 25 pounds per square inch. It has also been estimated that capillary blood pressure, when seated, is about 40 millimeters of mercury or about 0.8 pounds per square inch at these locations. If pressures higher than 40 millimeters of mercury are developed during sitting, the blood flow through the capillaries can be restricted. This restriction produces a much higher probability of the seated patient developing pressure sores.
In addition to pressure sores which result from pressure concentrations, many wheelchair seating systems are constructed in a manner producing shearing strain in the user's skin. This shear problem typically occurs when the patient's bones move relative to the patient's skin, for example, when the skin rests on a non-stretching seat surface and the patient's bones are shifted laterally relative to the stationary skin. Rubbing of the bones on the inside of the skin tissue can produce pressure sores, either alone or in combination with undesirable pressure concentrations. This problem is exacerbated when the patient has experienced muscle atrophy, since muscle tissue is no longer present to cushion relative movement of the bones and skin tissue.
Skin shear tends to exist whether or not a non-stretching wheelchair seating surface is contoured to match the body's shape. It is, however, much more pronounced when the non-stretching surface does not contour to the patient's body or when the non-stretching surface is pulled taught or hammocks under the patient's body. Hammocking is defined, as used herein, as a suspension of the bony prominences of the user or patient on the surface of a cushion or seat to thereby prevent total conformation of the cushion to the user's body, which in turn prevents complete pressure equalization.
Consequently, the need also exists to provide a seating system which, while anatomically contoured to the patient's body, also does not hammock or become pulled taught under the patient, thereby equalizing the support pressure and minimizing shear stresses on the patient's skin by allowing the skin to shift with or follow the patient's bones. Such seating systems will reduce the likelihood of pressure sore formation.
The successful treatment of pressure sores has typically required daily care and debridement for a period of many months. In some cases, surgical skin grafting is necessary. The problems and costs attendant to treating a single pressure sore are great, and therefore prevention of these sores must be considered a factor of prime importance in designing wheelchair seating systems.
The issue of seat-to-floor height is also a very important consideration in designing a proper seating system for wheelchairs. A common problem for wheelchair seat assemblies is that they are constructed in a manner which increases the seat-to-floor height, usually as a result of placing a pressure-relieving cushion on top of the existing wheelchair seat. Merely placing a bulky cushion under a wheelchair user, therefore, provides additional comfort at the expense of raising their seat-to-floor height. The issue of seat-to-floor height is very important to wheelchair users who are physically able to self-propel the wheelchair with their feet. These users need to self-propel their wheelchairs with one or both of their feet. Placing bulky cushioning under the seat of a wheelchair user raises their seat-to-floor height thus not allowing the user's feet to touch the floor or forcing the user to straighten their legs and lose their proper seating position in order to touch the floor. The straightening of the user's legs increases the difficulty of self-propelling the wheelchair. Raising the patient's seating height also can prevent the user from fitting underneath normal tables. Furthermore, some wheelchair cushions can be so large and heavy as to restrict the user's mobility and independence.
Consequently, there has been a long-standing need to provide a wheelchair seating system which provides the required comfort and support to the user, with particular emphasis on minimizing the formation of pressure sores, without excessively raising the patient's effective floor-to-seat height. Moreover, such comfort most preferably would be achieved while minimizing the bulk and weight of the wheelchair seating system.
2. Specific Prior Art Systems
Many prior art wheelchair seating systems are directed to only one or two of the problems above set forth. U.S. Pat. No. 5,074,620 to Jay, et al., for example, discloses a wheelchair seat system that is directed only to the problem of adjusting the height, pitch and roll of a wheelchair seat base or pan. U.S. Pat. No. 5,437,479 to Hutson is also directed solely to adjusting the height of a wheelchair seat. The issue of better weight distribution and the reduction of pressure concentrations in the ischia and coccyx areas is not directly addressed in either of these systems.
U.S. Pat. No. 5,088,747 to Morrison is illustrative of an approach which emphasizes pressure distribution at the expense of seat-to-floor height, bulk, weight and complexity. A vertically stacked assembly of a board-like support member, a foam member, a gel envelope and a second foam member are all placed on top of a standard wheelchair sling seat. This assembly is undoubtedly effective in reducing pressure concentrations, but its seat-to-floor height, bulk and weight are highly undesirable.
U.S. Pat. No. 4,953,913 to Graebe and sold under the trade name "ROHO" by Roho, Inc. of Belleville, Ill. employs a combination of an anatomically contoured relatively thin rigid seat pan or base with a pneumatic, multi-celled seat cushion. The seat pan is mounted over a standard sling seat and cannot be mounted to a wheelchair independent of the existing sling-seat, and accordingly increases the seat-to-floor height somewhat. The pneumatic multi-celled cushion tends to be relatively effective in distributing pressure, but such cells are well known to lack the desired degree of lateral stability and have substantial height. Thus, the increased height and lateral instability problems remain in the Graebe seat assembly.
In U.S. Pat. No. 4,629,246 to Fulton, a wheelchair seat assembly is disclosed which has a mounting structure which overcomes the seat-to-floor problem. Thus, a seat pan or base is provided which is hung or dropped down below the wheelchair seat-supporting frame members to accommodate a bulky, soft, seating cushion while maintaining the original seat-to-floor height. The seat base is notched to avoid interference with wheelchair componentry, particularly the folding cross-members. Thus, pressure distribution is accomplished while maintaining the seat-to-floor height, but these goals are met at the expense of bulk, and the seating assembly does not provide for shear force reduction on the patient's skin.
A variety of wheelchair seat assemblies exist, however, which have modified the shape of the support cushion or padding of the wheelchair seat from the Fulton-type rectangular block to a more anatomically-shaped surface. The design of these anatomically contoured cushions acts to better distribute user weight and to provide the user with additional stability.
One example is found in U.S. Pat. No. 4,643,481 to Saloff, et al. which discloses a two-piece seat base formed with an anatomically contoured upper surface. A composite foam and fluid cushion is placed over the seat base. Both the base and the cushion are formed in the shape of a pair of wings surrounding a void over which the user's ischia and coccyx are to be positioned. The fundamental approach of this design is to relieve the pressure under the seat bones by redistributing it to the surrounding areas of the buttock and thighs. Unfortunately, this structure does nothing to support the user's ischia bones and coccyx, and its design may well serve to increase the shearing forces on the patient's skin. The user's seat-to-floor height, and the weight and bulk of the seat assembly, clearly are increased.
Another approach is to provide cushions with materials of different resiliency positioned at various areas under the seating area of the patient such that more support is provided at certain anatomical locations. U.S. Pat. No. 4,951,334 to Maier and U.S. Pat. No. 4,837,881 to Kondo, et al. both disclose seat cushions having portions with differing compression characteristics. Another similar type of seating cushion is described in the 1982 Vasio "PARA" Cushion Brochure of Preston Corporation of Clifton, N.J. The assembly of the Maier patent is not anatomically contoured, and positions the ischial bones and coccyx over a void (as was similarly found in the Saloff et al system discussed above); it provides no support whatsoever to this important region. Whereas the Maier, Kondo and "PARA" cushions all provide for greater rigidity and therefore support at critical anatomical locations, none of these systems addresses the problems which they create by employing greater cushion thickness, namely, the raised seat-to-floor height problem and additional bulk and weight.
In U.S. Pat. No. 3,987,507 to Hall, a three-layer foam cushion is employed which has three round foam inserts of lower density than the surrounding foam which is placed into recesses in a central foam layer. U.S. Pat. No. 4,753,480 to Morrell also discloses a foam pad having cutout sections wherein inserts of lower density are placed. Both of these patents do provide additional support around the critical ischial and coccyx anatomical areas; however, being large foam sandwiches, they do not solve the problem of added seat-to-floor height, nor bulk and weight problems.
Various wheelchair seating assemblies also exist which provide cushioning using fluid or air-filled pouches specifically positioned to support the patient's ischial area. For example, U.S. Pat. No. 4,796,948 to Paul, et al. discloses a support system for a wheelchair wherein an inflatable support bladder is located directly under the patient's ischial tuberosities. This support bladder is normally deflated whereby only minimal support pressure exists on this ischial area; however, a fluid may be supplied to inflate the support bladder causing a much greater proportion of the patient's body weight to be supported by this inflatable pouch. By alternating between high and low levels of inflation in the bladder, the patient's weight can be periodically redistributed, away from and back to the patient's ischial tuberosities, thereby combating the formation of pressure sores.
U.S. Pat. No. 4,930,171 to Frantz discloses a fluid-filled bladder imbedded into a foam pad. This bladder is positioned directly under the patient's ischial area. However, the area under the coccyx is not supported at all since both the foam and fluid parts of the cushion are cut out in this region. Since both Paul and Frantz employ a single compartment bladder in which the fluid or air moves freely, both Paul and Frantz are subject to excessive migration of the fluid, which can cause bottoming out of the user on the bladder as the fluid moves and the patient's weight is shifted from side-to-side.
Cushion assemblies also exist in which a fluid-filled envelope covers the entire surface of a relatively rigid and anatomically contoured foam tray. U.S. Pat. No. 4,726,624 to Jay, for example, discloses a seat assembly having a fluid envelope covering a shaped, semi-rigid, foamed tray.
This tray is shaped with a recess in the ischial coccyx area. The fluid pad is divided into two compartments which cover the entire tray. The tray has side rims which are sloped downwardly from the outer edge toward the ischial coccyx area recess thus urging the thixotropic fluid material in the envelope to flow toward this depressed area. U.S. Pat. No. 5,018,790 to Jay and U.S. Pat. No. 5,378,045 to Siekman, et al. both disclose seat cushions having similarly formed fluid-filled envelopes. The two Jay systems do not address the problem of raised seat-to-floor height of the patient, and fluid bladders which cover the entire seating tray tend to add undesirable weight to the seat assembly.
A system sold under the trade name "AKROS" is manufactured by Akros Manufacturing Inc. of Cloversville, N.Y. This system is composed of a fluid pouch substantially covering a rigid seat base. The fluid pouch is cut away similar to the Maier patent, such that the ischial tuberosities and coccyx are positioned over a void and receive no support.
U.S. Pat. No. 5,395,162 to Jay discloses a wheelchair seat assembly in which a fluid-filled pad is positioned over a contoured tray or cushion. This assembly does not include a seat base nor is any teaching included as to dropping the seat cushion or tray to maintain seat height. A compartmentalized pad, however, is shown which can extend over the entire area of the cushion or only the ischial-coccyx region.
Finally, a prior art wheelchair seat assembly employs a chambered fluid filled insert in a foam seating pad of a wheelchair cushion. U.S. Pat. No. 5,189,747 to Mundy, et al. discloses a seat cushion having a flat, relatively rigid "internal" base or pan on which a shaped foam cushion is supported. The internal seat pan, in turn, is mounted on another or "external" rigid base or seat pan, which includes mounting clips for mounting the assembly in a dropped position to accommodate the relatively bulky cushion assembly. This foam cushion assembly is formed with a recess or pocket to receive an insert which can take the form of either a visco-elastic foam or a fluid pouch. The insert is stuffed or accordion folded into the cushion recess or pocket. If a fluid filled pouch is used, it contains three chambers for the ischial tuberosities and the sacral or coccyx region. The limitation with this system is that, while it provides a fluid pouch support in the critical ischial and coccyx regions, the provision of anatomical support over the rest of the seat assembly is achieved by a bulky combination of shaped foams of differing density. Moreover, the double seat base or pan construction undesirably increases the seat assembly's weight.